This invention relates to the art of agglomeration and more particularly to the formation of pellets from a resin in the form of a non free flowing powder of polytetrafluoroethylene (PTFE) and other polymers and copolymers.
In the molding of various plastic articles, it is customary to feed or stock the mold by pellets. The pellets include a base resin, such as polytetrafluoroethylene, together with various fillers, colorants and the like which are desired in the final molded product.
It has been a continuing problem to convert PTFE in the relatively low bulk density and low flow powder forms available from resin manufacturers to a high density free flowing agglomerated pellet that will rapidly and easily feed to modern automatic molding machines. This is accomplished by use of a wetting agent (e.g. trichloroethylene, alkyl alcohols, water soluble alcohols plus water or the like) to wet the low bulk density, non free flowing PTFE polymer, followed by various forms of rolling or tumbling of the wet polymer to promote agglomeration with accompanying bulk density increase. Known agglomerating apparatus and methods include double cone blenders and inclined pans or discs to convert the powder forms of PTFE to pellets. The agglomerated pellets are then dried and/or baked with or without solvent recovery.
Another apparatus/method has also been used, prior to this invention, for PTFE resin agglomeration. The polymer from a resin manufacturer (e.g. Teflon 7A or Teflon 7C from DuPont or G170, formerly G570, from ICI) is wetted with ethanol containing 5%-10% water after blending with desired additives (e.g. glass, bronze, various fillers, carbon black, etc.) and mixed in a blender to thoroughly and uniformly wet the polymer. The wetted polymer is fed through a screen type hammer mill to form an initial pellet. These seed pellets are then fed into a slightly tilted, heated octagonal stainless steel tube (10 feet long by 141/2 inches across opposite flats) which is rotating at 10-40 rpm. Agglomeration of the seed pellets occurs over the 1-2 minute contact time as they pass through the tube. The tube is heated to prevent major sticking of the wet pellets to the walls of the tube and the tube is inclined from 0.1 to 6 degrees to cause essentially plug flow through the tube. The agglomerated pellets are then tray dried with solvent recovery and baked for up to 12 hours at up to 610 F. (321 C.) to further densify the pellet.
In use of this latter apparatus/process over several years, problems have been encountered in making consistent pellet products, particularly with respect to pellet flowability.
Flowability of agglomerated automatic molding products (pellets) is measured by a slit flow test, known in this art. Pelletized PTFE (both virgin and filled) is placed into a triangular hopper apparatus. When the hopper apparatus is actuated, one side of the hopper gradually moves (as in the manner of a sliding gate), thus defining a slit in the bottom of the hopper. This opening increases in size until about 50 grams of pellets have fallen through and into a pan. The same weight of pellets, which exhibit better flowability, will fall through a smaller slit opening. The slit width is used as an indicator of how well the pellet material will fill mold cavities at plastic article fabricators. Variations in pellet flowability are typical of most free flow products made by this process.
A study to determine causes for this variation was conducted. This study included a review of a small scale sample preparation unit using an inclined unheated rotating disc in place of the heated, octagonal tube in the larger scale unit. The significant observations of this process were:
(1) The product was improved over the large scale process in slit flow and in bulk density and reproducibility.
(2) The type of rolling action was different from that in the octagonal rotating tube process.
(3) The residence time in the rotating disc unit was 2-5 minutes normally, while it was much less in the octagonal tube unit.
Studies were then conducted on a large scale rotating octagonal tube to increase residence time (e.g. lower tube angle, higher tube speeds, lower feed rates, etc.). No significant improvement in level or reproducibility of slit flow and bulk density was obtained in spite of increased residence time to that of a small scale rotating disc unit.
An apertured plate or disc at the discharge end of the octagonal tube was then installed to determine if the disc would improve the rolling action in the tube, to be more like that of the rotating disc agglomerating apparatus. Unexpectedly and surprisingly the disc acted like a friction or hydraulic clutch to generate a motion essentially throughout the length of the octagonal tube, analogous to an ocean wave of the pellets rolling over themselves. Several sized openings in the disc at the end of the rotating tube were tried and evaluated. With a smaller opening, pellet flowability increased and bulk density increased, giving this process the capability of varying both slit flow and bulk density to suit various end use requirements by changing the size of the end plate opening. For a plate of about 18 inches square, round openings in the plate of 7, 9, 11, 11.5 and 13 inches were used.